This research program's major goal is to increase understanding of the control system that links retinal metabolic load with nutrient supply from the retinal circulation. In the experiments proposed here, two different methods of measurement of retinal blood flow, the fluorescent microsphere method and the particle tracking method, will be used to make blood flow measurements in rats during normal conditions and during photic stimulation of the retina. The specific aims of this study are 1) Modification and characterization of the particle tracking method for measurement of blood flow through the retinal circulation of the rat, 2) Modification and characterization of the fluorescent microsphere method for the measurement of blood flow through the retinal circulation of the rat, 3) Characterization of the effect of flicker stimulation of the retina on retinal blood flow, and 4) Determination of the role that action potential generation in the inner retina plays in triggering increased blood flow during retinal stimulation. The microsphere method for blood flow measurement is a technique in which labeled microspheres are injected into the left ventricle of an anesthetized rat and become embedded in the retina. After the animals are sacrificed, the retina is removed and wholemounted, and the spheres are counted using a fluorescent microscope. In the particle tracking technique, very small microspheres (roughly 2 um in diameter) are injected intravenously. These particles are imaged using a microscope with a fluorescence attachment. Images are recorded on sVHS videotape and blood velocities and flows are calculated. Once the variability and reproducibility of these measurement techniques are optimized for use in the rat retina, the effects of neural activity on retinal blood flow will be investigated. First of all, the increase in retinal blood flow that accompanies flicker stimulation will be characterized in the rat retina. In later experiments, the Na+/K+ pump blocker TTX will be used to block action potential generation in inner retinal neurons to see if this activity triggers the increase in retinal blood flow that occurs in response to retinal stimulation.